1. Field of the Invention
The present invention relates to a high transmission stigmatic mass spectrometer as used in secondary ion mass spectrometry (SIMS). SIMS is described by A. Benninghoven et al in "Secondary Ion Mass Spectrometry" in Chemical Analysis, vol. 86, John Wiley and Sons, section 4, pp. 329 to 664.
In a SIMS instrument, that part of the instrument which is located downline from the sample and from the secondary ion extracting devices forms a mass spectrometer, the spectrometry of which greatly differs from that of thermo-ionization spectrometers in that the secondary ions emitted generally display a far more appreciable energy dispersion which may typically be as great as 20 eV. Under these conditions it is advantageous, in these instruments, not to filter the energy particle beams so as to preserve the entire ion signal available, one of the expected performance characteristics of the spectrometer being that it should have efficient transmission for a determined mass resolution, the term "transmission" designating the part of the secondary beam that is accepted by the spectrometer and the term "mass resolution" designating the smallest difference in mass between two masses that are measured separately. However since, in any spectrometer, it is necessary to diaphragm the particle beam to obtain a fixed mass resolution, it may be intuitively thought that the greater the mass resolution required, the more limited will be the transmission of the beam and the weaker will be the signal available to carry out the measurement.
The mass dispersion is obtained by making the particle beam go through a magnetic field created by the magnet of a magnetic sector "MS". Each non-relativistic particle that goes through the magnet is then deflected along a circular path with a radius of curvature R.sub.m defined by a relationship having the form: EQU R.sub.m =mv/qB=(1/B).multidot.(2mV/q).sup.1/2 ( 1)
according to which B designates the magnetic field, m the mass of the particle, V the acceleration voltage of the particle, q its electrical charge and v its speed. However, the relationship (1) reveals a phenomenon of chromatic dispersion of the magnetic sector which may seriously restrict the mass resolution by the fact that the radius of curvature R.sub.m depends both on the acceleration voltage V and on the mass m and that the energy dispersions in the SIMS analyses are relatively great.
As is also explained in the work by A. Benninghoven, this problem is usually resolved by compensating for the chromatic dispersion of the magnetic sector by that of an electrical field created by a voltage applied between two electrodes between which the particle passes. Under these conditions, the particle is further deflected along another circular path, the radius of curvature R.sub.e of which verifies a relationship of the form: EQU R.sub.e =2V/Q/qE (2)
in which V represents the acceleration voltage of the particle, E the electrical field prevailing between the two electrodes and q the electrical charge of the particle. The relationship (2) shows that the radius R.sub.e depends on the acceleration voltage but not on the mass. This makes it possible to state that an electrical field disperses chromatically but does not disperse in terms of mass. Consequently, to make a device, at the output of which the paths have a deflection depending on the mass but not on the energy of the particle, it is usual to associate an electrostatic sector ES that creates an electrical field on the journey of the particle to a magnetic sector MS which creates a magnetic field. Naturally, in this association, the characteristics of the electrical and magnetic sectors and their arrangement should be such that the chromatic dispersions compensate for each other exactly so that the spectrometer thus obtained is achromatic. Again, according to the prior art, two types of spectrometer may usually be considered, depending on whether the achromatism takes place at a single point of the output axis of the spectrometer or takes place throughout the output axis of the spectrometer. In these spectrometers, the achromatic output plane of an electric or magnetic sector is the plane on which there is located the point from which the energy dispersed paths seem to have emerged. The achromatic input plane is symmetrical with that of the output if the sector is symmetrical. Also in these spectrometers, any particles having a difference in energy .sub.-- E and converging towards the achromatic plane always take a path that leaves on the axis, and the achromatism on the axis implies that the the achromatic output plane of the electrostatic sector ES and the achromatic input plane of the magnetic sector MS are conjugate.
A known arrangement to achieve the achromatism throughout the axis of a spectrometer is, for example, that of the instrument IMS3F, marketed and manufactured by the Applicant Firm CAMECA. A description corresponding to this instrument may be found in an article by M. Lepareur, Le micro-analyseur de second generation CAMECA module 3F (The Module 3F CAMECA Second-Generation Micro-Analyzer) in the Revue THOMSON CSF, vol. 12, No. 1, Mar. 1980.
The usefulness of this arrangement is that, in addition to achieving achromatism throughout the axis, it projects, on the image intensifier, the ionic image of the mass filtered sample.
However since, in addition to their deflecting properties, the electrostatic or magnetic sectors possess focusing properties that depend on the shape given to the electrodes of the ES and to the pole pieces of the MS, these sectors generally have no symmetry of revolution, and the convergence along the two directions Oy and Oz, normal to the optical axis, is not identical. However, in the IMS3 apparatus, the spherical electrodes of the electrostatic sector ES have a focal length fe that is equal in the directions Oy and Oz and the planes located on each side of the electrostatic sector ES, at a distance, from the input faces, that is equal to the radius of the electrostatic sector ES, are conjugate. Furthermore, the shape given to the input faces of the magnetic sector MS gives it an optical diagram equivalent to that formed by a doublet of lenses. As a result, the lenses have equal focal lengths fm for the two directions Oy and Oz, and the space between them is not identical in the two directions. An input slit is placed at a distance fe upline from the input face of the electrostatic sector ES; an output slit is positioned at a distance fm downline from the magnetic sector MS. These two slits are optically conjugate and play a roughly equivalent role. It is the setting of the input slot that determines the mass resolution. To make a selection, according to a chromatic criterion, of the particles to be taken into account by the analysis, an energy slit is positioned at a distance fe from the output face of the electrostatic sector ES. In normal operation, the illumination pupil of the secondary emission, also called the cross-over, is imaged on the input slit and, hence, on the output slit. The plane of the sample is located on a diaphragm located just at the input of the electrostatic sector ES. Because of the disparity, in the directions Oy and Oz, of the spaces between the doublet lenses of the equivalent diagram of the magnetic sector MS, a stigmator is needed in a plane close to the output slit to enable the projection of the stigmatic image of the plane of the sample.
In this apparatus, an optical coupling device is placed between the electrostatic sector ES and the magnetic sector MS, firstly, so as to compensate for the chromatic dispersion of the electrostatic sector by that of the magnetic sector and, secondly, so as to conjugate the planes of the input slit and output slit. As is also described in the above-mentioned article by M. Lepareur, the optical coupling device may be made by means of a single lens. Once the optical characteristics of the two sectors, namely the magnetic and electrostatic sectors MS and ES, have been defined, there is only one configuration, having the parameters of distance between the two sectors, position of the lens, and excitation of the lens, that enables both compensating for the chromatic dispersions and conjugating the output slit with the input slit.
In this configuration, the achromatic output plane of the ES and the achromatic plane of the MS are conjugate.
However, as in every spectrometer, the mass resolution .DELTA.M/M with respect to the width of the input slit and the characteristics of the sectors is determined by a relationship having the form: ##EQU1## where .DELTA.M/M is the resolution in mass.
.DELTA.Wys is the width of the Gaussian image of the input slit at the output plane of the spectrometer; PA1 K.sub.M is the coefficient of dispersion in mass of the magnet defined by dy=K.sub.M dM/M; PA1 Ky and Kz are the coefficients of the second order aberration of the spectrometer; PA1 .theta.ys and .theta.zs are the angular apertures of the beam at the output plane.
The relationship (3) expresses the fact that the angular apertures along the orthogonal directions OY and OZ in the plane of the slits produce second order aberrations in the direction OY which is the direction of the mass dispersion.
Since the quantity of ions taken into account by the analysis is proportional to the product Wys x .theta.ys, there should certainly exist an optimum of the pair (Wys, .theta.ys) that minimizes the differences between two masses that can be resolved resolution in mass. However, there is every reason to reduce the aperture .theta.zs by optical means, it being understood that, as a result, the image Wzs is enlarged, but that this image has no effect on the resolution in mass.
An attempt to reduce the aperture of the beam at Z has been made on an Australian instrument known as SHRIMP where a four-pole lens, positioned before the electrostatic sector ES, squeezes the beam at Z. A description of this apparatus is given in an article by S. Clement, W. Compston, G. Newstead, "Design of a Large, High Resolution Ion Microprobe" in A. Benninghoven ed. Proceedings of the International Conference on SIMS and Ion Microprobes, Springer Verlag, 1977. But, in this instrument, it is not possible to project the image of the analyzed sample after the spectrometer.